This invention relates to a tunnel kiln for the firing of ceramic ware wherein movable carriers for the transport of ceramic ware are moved at the bottom of the tunnel kiln through a firing tunnel.
German DT-OS 2355 555 discloses such a tunnel kiln wherein movable carriers are moved through a tunnel kiln by means of sliding carriers which rest upon slide rails or slide bars. Sand seals affixed to the tunnel kiln are intended to prevent the heat of the firing tunnel from escaping to the outside. The firing tunnel which is open underneath and which is sealed from the outer air by the sliding carrier is intended to provide sufficient cooling of the slide rails and of the sliding carriers. This cooling effect is supposed to be sufficient for short transit times, but for longer transit times cooling means, particularly ventilators, are disposed at the open bottom of the tunnel kiln in order to keep the slide rails and slide carriers cool.
A disadvantage of these well known tunnel kilns is that the carriers can only be fired from above. Accordingly, since even distribution of heat is a requirement, a prolonged transit time in the firing tunnel is required.
Furthermore, it is a disadvantage that the carriers must be made out of steel which is protected by insulating material. Because the carriers are intensely heated during the transit through the firing tunnel and also are exposed to high temperatures in the area of the sand seals, such steel structures are subjected to increased temperature stresses. Therefore such steel structures undergo warping and deformation which effect a shortening of the life expectancy of the carriers.
Furthermore, the insulating means are characterized by poor stability when subjected to temperature changes and are subject to poor wear stability.
The transit time in which a carrier traverses the firing tunnel depends on the thickness of the insulating means. Thinly held insulation means thus cause a short transit time in order to prevent considerable destruction of the carriers. The movement of the carriers through the kiln cannot be interrupted when interruptions in the production of ware to be fired occur. Therefore, carriers without firing ware are introduced in the kiln in order to keep the transit times constant. It is quite obvious that heat losses occur because the ware-free carriers absorb the heat.
Due to the lateral insulation between the carrier and kiln wall the width of the firing tunnel is so reduced that only one train of carriers can traverse the kiln. The width and length of the carrier are limited because the carrier tend to warp under the influence of heat. Furthermore, the tunnel kiln is leaky because the carriers warp and the sand seals often do not exclude all the outer air, so that ambient outer air is drawn in at leaky spots, for example, where two carriers abut or between the carriers and the tunnel wall. That causes loss of heat.
For the return transport of carriers from the exit end to the entrance end expensive by-passes are required to be built.
Another disadvantage of the heretofore known tunnel kilns is that lubrication has to be applied between the slide bars and slide runners, necessitating the introduction of lubrication stations at appropriate distances. The maintenance of these is also expensive.
Accordingly, one object of the present invention is to overcome the aforesaid disadvantages of known prior art tunnel kiln arrangements and to provide a tunnel kiln with a carrier movable through the firing tunnel wherein the temperature is advantageously distributed through the whole firing tunnel, and wherein the firing tunnel is completely insulated from the outer air. Furthermore, the carriers to be transported through the firing tunnel are made without temperature sensitive steel parts and lubrication is not required.
These objectives are achieved, according to the invention by providing a tunnel kiln having an oven floor with a closed botton and a stationary floor flue with at least one transport way arranged on the floor flue, the transport way serving to move the carriers through the firing tunnel, the transport way consisting of support plates and rolls or balls which roll upon the support plates with the carriers being supported by the rolls or balls.
The fact that the tunnel kiln is provided with a closed bottom and a stationary disposed bottom flue permits the firing tunnel to be heated by burners operable in the vicinity of the bottom flue and by burners functioning directly inside the firing space. This results in an advantageous distribution of heat in the firing tunnel which has an advantageous influence upon the transit time of the carriers through the tunnel kiln. The stationary bottom flue also permits the constant introduction into the free space near the flue supports of the combustion gases of the burners, operable within the area of the bottom flues, so that the combustion gases may rise upwardly all over the breadth of the firing tunnel.
No heat losses by drawing in external air can occur because the tunnel kiln is completely free of leaks.
The carriers may be left safely inside the tunnel kiln without cooling, according to the firing time of the goods, and no damage to the carriers occurs as the carriers consist of fireproof or heat resistant material which is stable against changes of temperatures and do not contain any temperature sensitive steel parts.
That is particularly advantageous when the production of the firing ware is interrupted and the feeding of the new ware lags. The tunnel kiln may now be shut at its highest temperature and will be ready for production even after several hours inactivity.
The dense, airtight construction of the tunnel kiln makes it possible to build the firing tunnel at an arbitrary width so that it may accept several transport ways side by side. The particular arrangement of the transport ways and of the carriers, and the cooperative arrangement of these allows always an optimal support for the firing ware.
Applicant's invention provides for reducing the pushing power required to move the carriers through the tunnel kiln as much as possible while preventing oblique movements of the carriers. This objective is achieved by furnishing the supporting plates and the carriers with grooves within which the balls or short rolls move. The grooves are positioned at the longitudinal faces of the support plates and carriers, the longitudinal faces running in the direction of the firing tunnel.
The rolls whose dimensions correspond to that of the balls, roll in the grooves with very low friction, so that a lower pushing force is able to move the carriers through the tunnel kiln.
Due to the fact that the balls or rolls are guided in the grooves, an oblique running of the carriers cannot occur, and the disadvantages resulting therefrom are precluded.
The small size of the balls and the short rolls causes an improved stability against changes of temperature. The balls and the rolls may be used for prolonged periods of time without requiring to be replaced.
The grooves for the balls may be semi-circular or angular and the grooves for the rolls may be oblong. Additional advantages of the invention are that the supporting plates in the area of each carrier-groove may be replaced by two pipes made of a fire resistant material which extends through the tunnel kiln, the two pipes serving as rolling surfaces for the balls and being disposed at a distance from each other and in recesses of the cover of the bottom flue.
The use of pipes as rolling surfaces is very economical because they are simpler to manufacture than supporting plates which are usually prepared by an extrusion process. Pipes also use less material than plates.
The pipes offer another advantage because a turn along their longitudinal axis creates a new running surface for the balls when so needed due to wear.
The material rubbed off the running surfaces of the pipes does not remain where it occurs but falls down between the pipes because the pipes are disposed at a distance from each other. This prevents prolonged damage to the pipes.
Additional work which is required when an automatic operation is desired, is small because the balls are easily transported in an inclined groove from the exit end of the kiln to the entry of the kiln in order to be reused again.
No by-passes are needed to transport the carriers from the exit end to the entrance end of the tunnel kiln. The carriers can be transported one on top of the other. Likewise there is no need for large spaces to store the carriers because they are easily stacked.
Another advantage is the small need for maintenance because no lubrication stations are needed.
Other features which are considered characteristic of the invention are set forth in the appended claims.
Although the invention is illustrated and described in relationship to specific embodiments, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.